1. Field of the Invention
The present invention concerns an oxygen scavenger for a boiler water system and a method of removing dissolved oxygen in a boiler water system and, more in particular, it relates to an oxygen scavenger for a boiler water system which is highly safe to a human body and does not result in the reduction of concentration in a dissolving tank, that is, a chemical for scavenging dissolved oxygen in a boiler water system and a method of removing dissolved oxygen in a boiler water system by using the chemical.
2. Description of the Prior Art
Oxygen dissolved in water causes corrosion of metal materials to shorten the life of various kinds of apparatus (for example, boiler and heat exchanger) or pipelines. In view of the above, an oxygen scavenger has been blended and used in various kinds of boiler feedwater systems for removing dissolved oxygen in water. Heretofore, hydrazine has been used most generally as the oxygen scavenger. Since reaction products of hydrazine with oxygen is nitrogen and water, it has an advantageous merit of not increasing the dissolved solid matters in the boiler water and enabling concentration control of hydrazine. However, since there is a worry of causing undesired effects on a human body, it has not been used to a system using raw steams. On the other hand, there is a sodium sulfite which is authorized food additives as a highly safe oxygen scavenger but, since it reacts too rapidly with oxygen, it has a drawback of causing reduction of concentration in a chemical dissolving tank and rather promoting corrosion in the boiler tube.
On the other hand, starch has been used long since as a corrosion inhibitor use in boilers.
Further, an aqueous oxygen scavenging composition comprising 100 parts by weight of an alkali metal sulfite and 0.05 to 20 parts of dextrin added thereto has been proposed (Japanese Patent Laid Open Sho 57-7225), in which dextrin is blended for improving the stability of the sulfite and the oxygen scavenging effect is provided by the sulfite.
Although starch has been used as the boiler corrosion inhibitor, it has not been known as an oxygen scavenger. In addition, natural starch has a drawback that the viscosity is high and the handling is not easy.
It is possible to increase DE (dextrose equivalent: reducing sugar is measured as glucose and DE is defined as a ratio of the reducing sugar to solid content) and make it into a cold water soluble starch by acidolysis, enzymolysis and oxidative decomposition. However, if the thus obtained starch is added as it is to boiler water as an oxygen scavenger, since it greatly consumes P-alkalinity in the boiler water, there is a problem that the combined use of the alkali agent is inevitable. On the other hand, in the composition as described in Japanese Patent Laid Open Sho 57-7225, since the oxygen scavenger as the main ingredient is alkali metal sulfite, the foregoing corrosion problem in the boiler has not yet been dissolved.